Recently, there has increasingly been desired for the development of a technique which permits the more finely processing step with respect to the thin film-forming technique used in the field of the semiconductor and this results in the occurrence of a variety of related problems.
In an example of the technique for forming electrical connections of a thin film in a semiconductor device, copper has mainly be used as a material for the electrical connection because of its low resistivity. However, it is technically difficult to etch copper and copper may easily penetrate or diffuse into the underlying layer such as an insulating film and accordingly, a problem arises such that the reliability of the resulting device is lowered.
To solve this problem, such diffusion of the copper has conventionally been prevented by forming a metal thin film (or a conductive barrier film) on the inner wall surface of the interlayer-connecting holes in a multi-layered electrical connection structure according to, for instance, the CVD technique; and then forming a layer for making the electrical connections by the application of a copper thin film on the conductive barrier film so that the resulting copper thin film never comes in direct contact with the underlying insulating film such as a silicon oxide film.
In this case, it has been required that fine contact holes, trenches or the like each having a high aspect ratio should be plugged or filled up with a thin barrier film while ensuring a high rate of step-coverage, with the foregoing demands for the use of electrical connections having a multi-layered structure and a further miniaturized pattern.
Under such circumstances, there has been proposed, for instance, a method for forming a barrier film having a desired thickness, according to the ALD technique which comprises the steps of raising the temperature of a substrate introduced into a vacuum chamber to a predetermined level; introducing one of a nitrogen atom-containing gas and a high-melting metal-containing gas into the chamber to thus make the same adsorb on the substrate; vacuum-evacuating the same gas; then introducing the other gas into the chamber to thus make them react with one another on the substrate; vacuum-evacuating the other gas introduced; and repeating the foregoing steps to thus form, on the substrate, a laminate of a plurality of metal nitride thin films each having a thickness roughly corresponding to one atom (hereunder referred to as “mono-atomic layer”) (see, for instance, Japanese Un-Examined Patent Publication Hei 11-54459 (for instance, Claim 1)).
Moreover, there has also been known a method for forming a barrier layer, which comprises the step of depositing a layer of a material such as Ta, TiN or TaN using, for instance, the ALD technique (see, for instance, Japanese Un-Examined Patent Publication 2004-6856 (Claims and the like)).
The foregoing ALD technique is similar to the CVD technique in that it makes use of a chemical reaction between two or more kinds of precursors. However, these techniques differ from one another in that the usual CVD technique makes use of such a phenomenon that the different kinds of precursors in their gaseous states come in close contact with one another to thus make them chemically react with one another, while the ALD technique makes use of a surface reaction between the different kinds of precursors. More specifically, the ALD technique comprises the step of supplying a kind of precursor (for instance, a reactant gas) onto the surface of a substrate on which another kind of precursor (such as a raw gas) has been adsorbed in advance to bring these two kinds of precursors into contact with one another and make them react with one another on the surface of the substrate and to thus form a desired metal film. In this case, the reaction between the precursor initially adsorbed on the substrate surface and the precursor subsequently supplied onto the surface proceeds, on the substrate, at a quite high rate. The precursors usable herein may be in any state such as a solid, liquid or gaseous state and the raw gas is supplied while using a carrier gas such as N2 or Ar. As has been discussed above, this ALD technique is a method for forming a mono-atomic thin film by repeating the step for adsorbing the raw gas on the substrate and the step for making the adsorbed raw gas react with the reactant gas alternatively. In other words, this technique can ensure an excellent rate of step coverage since the adsorption and further this technique the reaction always take place within the superficial dynamic region and permits the improvement of the density of the resulting film since the raw gas and the reactant gas are reacted with one another while separately introducing them into the reaction zone. For this reason, this technique has become of major interest lately.
The conventional mono-atomic layer-deposition apparatus (ALD apparatus) for forming a thin film according to the foregoing ALD technique consists of a film-forming apparatus provided with a vacuum evacuation means and the film-forming apparatus further comprises a substrate-mounting stage equipped with a heating means and a gas-introducing means arranged on the ceiling of the film-forming apparatus, which is opposed to the substrate-mounting stage. As an example of such an ALD apparatus, there has been known one having such a construction that a desired raw gas and a reactant gas are introduced into the apparatus through the gas-introducing means while setting a predetermined time lag between their introduction times to thus repeatedly carry out the raw gas-adsorption step and the reaction step in which the raw gas is reacted with the reactant gas by the aid of the plasma for the preparation of a thin film having a desired thickness (see, for instance, Japanese Un-Examined Patent Publication 2003-318174 (Claims and the like)).